1. Field of the Invention
The present invention relates to an optical apparatus for effectively illuminating a surface of a predetermined shape, and, particularly, it relates to a mask illuminating system for use in an apparatus for printing the pattern of a semiconductor integrated circuit.
2. Description of the Prior Art
In recent systems for printing the fine pattern of an integrated circuit, a mirror projection optical system having the imaging magnification of one-to-one has been utilized. Such a mirror projection optical system has an arcuate slit-shaped superior imaging area. Thus, an illuminating optical system to be used when the printing of an integrated circuit pattern is carried out by the use of such mirror projection optical system must similarly illuminate the arcuate slit-shaped area more effectively. An illuminating optical system suitable for such a purpose in disclosed in U.S. Pat. No. 4,294,538, for example. As shown in FIG. 1 of the drawings attached to the subject specification, such illuminating optical system comprises three concave spherical mirrors 1, 2 and 3, the spherical centers of which are disclosed on a common optical axis 6. A slit plate 4 is positioned perpendicular to the optical axis 6 with the slit-like opening 4 being formed into an arcuate shape as seen from FIG. 2. A point source of light 5, which may be a super-pressure mercury lamp or the like, is located on the optical axis 6 such that the beam of light therefrom is reflected by the spherical mirror 1 and then converged at the opening of the slit plate 4 to form an arcuate light source image. After passing through the opening of the slit plate 4, the arcuate beam is returned to its original point form by the spherical mirror 2 and then reflected by the spherical mirror 3 to form a slit-like image 7. Such an optical system provides an effective and convenient means for utilizing a point source of light to form an arcuate slit-shaped illuminating area. However, due to the aberrations at the spherical mirrors 2 and 3, the image of the slit opening would be blurred. In order to ensure that the mirror projection optical system for printing the pattern operates at its maximum performance, it is necessary to suppress the occurrence of the image blur sufficiently.